The present invention relates to a milling cutter employed for milling work and particularly to protecting the milling cutter from accidental detachment during cutting.
It has generally occurred that a milling cutter, such as an end mill or the like, which is attached to a milling chuck for use, while being under intensive stress exerted towards being detached from the milling chuck during cutting operation, becomes loosened off by resisting the gripping strength of the milling chuck, which results in excessive cutting in the workpiece and finally in detaching from the milling chuck. It is known in the prior art to provide a milling cutter 1, as shown in FIG. 6, which has an external thread 2a in the end of a shank 2 to prevent accidental detachment. The milling cutter 1 is attached through a sleeve 4 and a collet 5 to the inside of a chuck body 3 and positioned in the direction of insertion such that the end face 2b touches a raised center part 3a on the chuck body 3, wherein the external thread 2a tightly fits an internal thread of the collet 5 by mating so as to prevent detachment and also, the collet 5, when the milling cutter 1 rotates against the collet 5, will move in the outward direction so as to securely hold the shank 2 of the milling cutter 1 by means of a tapered contacting area thereof being against the sleeve 4. It appears that, because a straight part of the shank 2 in the prior art milling cutter 1 is shortened by sparing for the external thread 2a, the effective shank length for gripping effectively becomes shorter as compared with the entire shank length, and hence, the gripping area for transmitting rotary torque will be reduced, which may cause a tendency of slipping since the gripping strength is decreased and also a common occurrence of so-called trembling, resulting in incorrectness of the machining accuracy. Thus, a specific chuck whose construction is complicated in structure,as shown in FIG. 6, has been required for use, in which the adjustment of the milling cutter 1 for positioning towards the axis thereof cannot be made against the chuck body 3. As shown in FIG. 7, when a milling cutter 6 with a small diameter, similar to the one as set forth above, is gripped by a prior art milling chuck, a collet 8 which includes an internal thread part 8a for mating with an external thread part 7a of the milling cutter 6 and another internal thread part 8b for accepting a bolt 9 that faces the end face 7b is required, wherein the disadvantages are such that the effective shank length is short and a construction of the collet 8 is somewhat complicated. Additionally, the external threads 2a, 7a provided respectively in the milling cutters 1, 6 of the prior art are exposed such that damage to the threads 2a, 7a may be inevitable in falling or collision, whereby will result in high disposal rates of such expensive milling cutters 1, 6. Accordingly, great care should be taken to handle and maintain the milling cutters 1, 6, whereby the efficiency of working may be decreased.
By any possible means, it is known in the prior art to offer the problems associated with the milling cutters 1, 6 having the external threads 2a, 7a respectively, and thus, wherein the effective shank length is shortened such that the gripping strength and rigidity may be decreased and handling may be not as much as with a milling cutter having a straight shank without any external thread.